1. Field of the Invention
The present invention relates generally to graphics processing and methods relating thereto. More particularly, the present invention relates to a computer graphics processing system, computer memory, and related methods that utilize a hierarchical image depth buffer.
2. Description of the Related Art
The process of rendering image data within computer graphics systems is computationally intensive and complex. One significant limitation on the rendering process involves the vast amount of memory bandwidth required due to the volume of data accessed from memory. While there are various segments of memory used during the graphics rendering process, a significant memory bottleneck involves processing data for a memory buffer known as the Z buffer. The Z buffer contains depth values for each of the pixels in a two-dimensional projection of a three-dimensional scene. A particular portion of the rendering process that creates high Z buffer memory traffic is referred to as pixel visibility. This is the process of mapping object data representing various three-dimensional objects to be rendered, to a two-dimensional image to be displayed. The two-dimensional image to be displayed is typically stored in a frame buffer. While rendering the three-dimensional objects, the computer graphics system determines particular pixels for those objects that are visible for a user at a desired viewpoint. For example, one object may be behind another object so that the object furthest from the viewpoint is not visible. The depth data found within the Z buffer is typically used as part of this pixel visibility determination process. However, due to the increasing complexity of scenes and images used in computer graphics applications, such as fast-motion video gaming software, efficient use of Z buffer memory bandwidth has become increasingly important. In certain applications, more memory bandwidth is consumed by Z buffer accesses than any of the other types of memory accesses involved in 3D rendering.
One prior method and system relating to Z buffers and the visibility determination process is described in U.S. Pat. No. 5,949,423 (“the '423 patent”). The method described in the '423 patent identifies a process of sorting certain bounding volumes into an order for testing. While sorting may improve certain aspects relating to depth value usage, sorting also requires significant additional computation time and complexity. Added computation time and complexity is disadvantage in computer graphics methods. It would be desirable to improve Z buffer memory bandwidth efficiency without requiring the added computation and complexity involved using sorting methods. Accordingly, there is a need for an improved graphics processing system and method to perform image rendering with an image depth buffer (Z buffer).